Medical device electrical lead design for preventing transmittance of unsafe currents to a patient

ABSTRACT

An electrical lead including a conductor assembly, an electrode, and a thermally sensitive material. The conductor assembly has one or more conductors. The electrode is in electrical communication with one of the conductors and has an outer contact adapted for contacting adjacent body tissue of a patient. The thermally sensitive material is electrically connected between the one conductor and the electrode outer contact, and is configured to exhibit high impedance in the presence of currents considered unsafe to the patient, thereby preventing the unsafe currents from flowing through the thermally sensitive material and through the electrode outer contact potentially causing the adjacent body tissue to increase in temperature to an unsafe level. The unsafe currents cause the thermally sensitive material to increase in temperature, thereby causing the material to transition to a high impedance state.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority to U.S. provisional patentapplication filed Mar. 9, 2007 and assigned Ser. No. 60/894,135, theentire disclosure of which is incorporated herein by reference.

FIELD

The present invention relates generally to medical devices, and, moreparticularly, relates to designs for medical device electrical leadsextending between the medical devices and the patient.

BACKGROUND

Since the introduction of the first implantable pacemakers in the 1960s,there have been considerable advancements in both the fields ofelectronics and medicine, such that there is now a wide assortment ofcommercially available body-implantable electronic medical devices. Thisclass of implantable medical devices (IMDs) generally includestherapeutic and diagnostic devices, such as pacemakers,cardioverter/defibrillators, hemodynamic monitors, neural stimulators,and drug administering devices, as well as other devices for alleviatingthe adverse effects of various health ailments.

As is known, modern electrical therapeutic and diagnostic devices forthe heart and/or other areas of the body generally include an electricalconnection between the device and a patient's body. This connection isusually provided by at least one medical electrical lead, which istypically implanted (at least partially) within the patient's body. Forexample, a neural stimulator delivers mild electrical impulses to neuraltissue using one or more electrical leads. Such neural stimulation oftenresults in pain relief or a reduction in tremors depending on where theelectrodes are placed. Each electrical lead used with such devicestypically takes the form of a long, generally straight, flexible,insulated set of conductors. At its proximal end, the lead is typicallyconnected to a connector of the device, which also may be implantedwithin the patient's body. Generally, one or more electrodes are locatedat or near the distal end of the lead and are attached to, or otherwisecome in contact with, the patient's body. Such devices may be controlledby a physician or the patient through the use of an external programmer.

Other advancements in medical technology have led to improved imagingtechnologies, e.g., magnetic resonance imaging (MRI). As furtherdescribed below with respect to its process, MRI is an anatomicalimaging tool which utilizes non-ionizing radiation (i.e., no x-rays orgamma rays) and provides a non-invasive method for the examination ofinternal structure and function. In particular, MRI permits 3-D imagingof soft tissue better than any other imaging method. During the MRIimaging sequence, a radio-frequency field is applied to the patient.Magnetic resonance spectroscopic imaging (MRSI) systems are also knownand are herein intended to be included within the terminology “MRI”systems or scanners.

Further, shortwave diathermy, microwave diathermy, ultrasound diathermy,and the like have been shown to provide therapeutic benefits topatients, such as to relieve pain, stiffness, and muscle spasms; toreduce joint contractures; to reduce swelling and pain after surgery; topromote wound healing; and the like. Generally, in using such diathermyapparatuses, energy (e.g., short-wave energy, microwave energy,ultrasound energy, or the like) is directed into a localized area of thepatient's body.

Traditionally, use of the above-described technologies have beendiscouraged for patients having IMDs, as the environment produced by theMRI or diathermy apparatuses is generally considered hostile to suchIMDs. As is known, the energy fields, generated during the MRI ordiathermy processes, have potential for inducing an electrical currentwithin leads of IMDs as well as leads of other medical devices locatedwithin the patient. In conventional leads, this electrical current istypically conducted into tissue adjacent to the ends of the lead.Because the tissue area adjacent to the electrodes is often very small,the current conducting through this adjacent tissue results in thetissue heating. This may result in tissue damage when the currents aretoo large.

Thus, what are needed are medical device electrical lead systems thatreduce tissue heating to levels that do not induce tissue damage.

BRIEF SUMMARY OF THE INVENTION

In one embodiment, an electrical lead is provided. The electrical leadincludes a conductor assembly, an electrode, and a thermally sensitivematerial. The conductor assembly has one or more conductors. Theelectrode is in electrical communication with one of the conductors andhas an outer contact adapted for contacting adjacent body tissue of apatient. The thermally sensitive material is electrically connectedbetween the one conductor and the electrode outer contact, and isconfigured to exhibit high impedance in the presence of currentsconsidered unsafe to the patient, thereby preventing the unsafe currentsfrom flowing through the thermally sensitive material and through theelectrode outer contact potentially causing the adjacent body tissue toincrease in temperature to an unsafe level. The unsafe currents causethe thermally sensitive material to increase in temperature, therebycausing the material to transition to a high impedance state.

In another embodiment, an electrode is provided. The electrode includesan outer contact, an inner contact, and a thermally sensitive material.The outer contact is adapted for contacting adjacent body tissue of apatient. The inner contact is adapted for electrical coupling to a leadconductor. The thermally sensitive material is electrically connectedbetween the inner contact and the outer contact, and is configured toexhibit high impedance in the presence of currents considered unsafe tothe patient, thereby preventing the unsafe currents from flowing throughthe thermally sensitive material and through the outer contactpotentially causing the adjacent body tissue to increase in temperatureto an unsafe level. The unsafe currents cause the thermally sensitivematerial to increase in temperature, thereby causing the material totransition to a high impedance state.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an exemplary IMD as provided in apatient in accordance with certain embodiments of the invention.

FIG. 2 is a perspective view of another exemplary IMD as provided in apatient in accordance with certain embodiments of the invention.

FIG. 3 is a perspective view of a further exemplary IMD as provided in apatient in accordance with certain embodiments of the invention.

FIG. 4 is a perspective view of a medical device electrical lead inaccordance with certain embodiments of the invention.

FIG. 5 is a cross sectional view of a ring electrode of the medicaldevice electrical lead of FIG. 4, taken along the lines V-V inaccordance with certain embodiments of the invention.

FIG. 6 is a perspective view of the medical device electrical lead ofFIG. 4 in accordance with certain embodiments of the invention.

FIG. 7 is a plot exemplarily showing a resistance versus temperaturerelationship for a typical positive temperature coefficient (PTC)material.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following detailed description should be read with reference to thedrawings, in which like elements in different drawings are numberedidentically. The drawings depict selected embodiments and are notintended to limit the scope of the invention. It will be understood thatembodiments shown in the drawings and described below are merely forillustrative purposes, and are not intended to limit the scope of theinvention as defined in the claims.

Embodiments of the invention relate to implanted devices, andspecifically relate to designs for medical device electrical leadsextending between the implanted device and the electrodes of theimplanted leads. In particular, the lead designs are configured forpreventing unsafe currents from being conducted through the electrodeand into the tissue of the patient. Embodiments described andillustrated herein pertain to implantable medical devices (IMDs);however, the invention can extend to any lead-bearing medical device,whether implantable or not. Furthermore, while the embodiments providedherein relate to certain IMDs, it should be appreciated that suchembodiments are exemplary in nature. As such, the invention is notlimited to any particular IMD, but instead is applicable to any IMD,including therapeutic and diagnostic devices, such as pacemakers,cardioverter/defibrillators, hemodynamic monitors, neurostimulators, anddrug administering devices, as well as other devices for alleviating theadverse effects of various health ailments.

FIG. 1 illustrates an exemplary IMD in accordance with certainembodiments of the invention. The IMD 10 shown is a typical spinal cordstimulation (SCS) system and includes a pulse generator such as a SCSneurostimulator 12, a lead extension 14 having a proximal end coupled tothe neurostimulator 12, and a lead 16 having a proximal end coupled to adistal end of the extension 14 and having a distal end coupled to one ormore electrodes 18. The neurostimulator 12 is typically placed in theabdomen of a patient 20, and the lead 18 is placed somewhere along thepatient's spinal cord 22. While only shown with a single lead 18, it isto be appreciated that the IMD 10, in certain embodiments, can have aplurality of leads. Such a system may also include a physicianprogrammer and a patient programmer (not shown).

The neurostimulator 12 may be considered to be an implantable pulsegenerator and capable of generating multiple pulses or other electricalwaveforms. While the neurostimulator 12 typically provides electricalstimulation by way of pulses, other forms of stimulation may be usedsuch as continuous electrical stimulation.

The lead 16 includes one or more insulated electrical conductors eachcoupled at their proximal end to a connector 24 and to the electrodes 18(or contacts) at its distal end. As is known, some leads are designed tobe inserted into a patient percutaneously and some are designed to besurgically implanted. In certain embodiments, the lead 16 may contain apaddle at its distant end for housing the electrodes 18. In alternateembodiments, the electrodes 20 may comprise one or more ring contacts atthe distal end of the lead 16.

While the lead 16 is shown as being implanted in position to stimulate aspecific site in the spinal cord 22, it could also be positioned alongthe peripheral nerve or adjacent neural tissue ganglia or may bepositioned to stimulate muscle tissue. Furthermore, electrodes 18 (orcontacts) may be epidural, intrathecal or placed into spinal cord 22itself. Effective spinal cord stimulation may be achieved by any ofthese lead placements. While the lead connector at proximal end of thelead 16 may be coupled directly to the neurostimulator 12, the leadconnector is typically coupled to the lead extension 14 as is shown inFIG. 1.

FIG. 2 illustrates another exemplary IMD in accordance with certainembodiments of the invention. The IMD 30 shown is a typical deep brainstimulation (DBS) system and includes substantially the same componentsas does an SCS; that is, at least one neurostimulator, at least oneextension, and at least one stimulation lead containing one or moreelectrodes. As can be seen, each neurostimulator 32 a and 32 b isimplanted in the pectoral region of patient 34. Corresponding extensions36 a and 36 b are deployed up through the patient's neck, andcorresponding leads 38 a and 38 b are implanted in the patient's brain40 as is shown at 42 a and 42 b. As can be seen, each of the leads 38 isconnected to its respective extension 36 just above the ear on bothsides of the patient 34.

FIG. 3 illustrates a further exemplary IMD in accordance with certainembodiments of the invention. The IMD 50 is a cardiac medical device,exemplarily shown as a pacemaker, and includes one or more leads 52implanted in a patient 54. The leads 52 extend from the pacemaker can 56and lead into the patient's heart 58 via a vein 60. Located generallynear distal ends 62 of the leads 52 are one or more exposed conductiveelectrodes 64 that are attached to the heart tissue for sensing cardiacactivity, delivering electrical pacing stimuli, and/or providing acardioversion/defibrillation shock to the heart 58. The contact areabetween the electrodes 64 and the tissue of the heart 58 may be verysmall as compared, for example, to the contact area between the IMD 50and the patient's body.

To date, there have been many designs with respect to medical deviceelectrical leads, such as the implantable leads of the IMDs exemplifiedin FIGS. 1-3 as well as implantable leads of other medical devices, soas to limit the penetration therein of significant alternatingelectromagnetic fields and/or radio-frequency energy. In particular, avariety of different coverings have been used for such leads to providesuch protection. However, it has been found that most of these coveringsdo not adequately protect tissue from heating. In turn, there continuesto be potential for currents being induced within the lead conductors(from such fields) which can cause thermal damage to patient tissue. Asdescribed above, these induced currents can be found to be harmful tothe patient. In addition, events other than the above-described inducedcurrent phenomenon can be sources for currents which can also be harmfulto the patient. For example, such events can stem from medical deviceequipment failure, defective medical device, etc. As such, a variety oflead designs have been taught, to be used solely or in combination withthe lead shield coverings, so as to prevent the passage of harmfulelectric currents to the patient from one or more of the eventsdescribed above.

For example, a number of lead designs have been taught to specificallyprotect the patient from potential detrimental effects of unsafecurrents induced from electromagnetic fields. In some cases, the leaddesigns involve additional circuitry so as to provide further electrodepaths for the induced current. In turn, the heat brought to the tissuefrom the dissipating current is spread over a larger portion of tissue,thereby decreasing the likelihood of tissue damage from any one area oftissue being exposed to the current. In related cases, the lead designsare configured to separate the higher frequency induced currents fromthe lower frequency therapy signals (e.g., for pacing, stimulation,sensing, and the like) signals. As a result, filtering of the inducedcurrents is achieved, while not interfering with the normal functioningof the medical device. In further lead designs, use of passive and/oractive electronic components are provided within the lead circuit toprevent or at least limit unsafe currents being induced fromelectromagnetic fields and/or stemming from other events, as exemplifiedabove. For example, lead designs employing passive components,specifically inductors, have been taught so as to function inattenuating the high frequencies of magnetic signals generally used forMRI. Alternatively, lead designs employing active components have beentaught so as to limit current flow upon detection of a predeterminedphysiological detection. For example, such designs can involve detectionof electromagnetic fields in a predetermined frequency range (via asensor), followed by activation of a CMOS Field Effect Transistor (FET)within the lead body so as to open the lead connection to the electrode.

The above lead designs have been shown to have varying degrees ofeffectiveness; however, all can have drawbacks. For example, by usingadditional circuitry and/or electrical components, there is increasedcost in manufacturing the lead as well as increased sources for leadfailure. Additionally, the control inputs to such devices can also beaffected by these fields. As such, they are limited with respect totheir application in protecting the patient due to other events causingunsafe currents, including medical device equipment failure, defectivemedical device, etc.

A number of lead designs have been taught that are more universallyapplicable with respect to preventing or limiting unsafe currents,regardless of whether they are induced from electromagnetic fieldsand/or radio-frequency energy or are the result of other events causingunintended unsafe currents, as exemplified above. Specifically, theselead designs are designed to include electronic components that aregenerally passive, similar to that already described above with respectto use of an inductor. In turn, based on the magnitude of what issensed, the device is configured to automatically respond to modify itsoutput. Some of these devices have involved those designed for currentstoppage, such as micro-sized fuses or circuit breakers, and are gearedto interrupt the electrical connection to the corresponding electrodesupon unsafe current being sensed by the current stoppage devices. Otherdevices have involved those designed to limit current. In particular, adiode has been taught to be used. For example, diodes are commerciallyavailable which would block current exceeding certain current levels.

However, once again, these lead designs have limitations. For example,with respect to the current stoppage devices, once tripped, such leadscan no longer facilitate monitoring or therapy functionality. As such,while the devices enable the current to be entirely prevented fromflowing to the patient, if the patient warranted diagnosis or therapyfrom the lead following such device being tripped, there would be littlerecourse without first replacing components within the devices (as inthe case of fuses) or resetting the devices (as in the case of circuitbreakers). With respect to the current limiting designs, the device canbe used following a high voltage event without replacement of componentsof the device or resetting of the device. However, such a lead designstill allows for current, even at reduced levels, to pass to the patientvia the electrode during such high voltage events.

Thus, embodiments of the invention involve lead systems that function inlimiting the passage of unsafe electric currents through a medicalelectrode attached to a patient, while further overcoming one or more ofthe limitations facing the lead designs taught to date. In particular,the lead systems provide such functionality using a limited amount ofadditional materials, while also being widely applicable so as to beused for unsafe currents which are induced from electromagnetic fieldsor stem from other events, as exemplified above. In addition, these leadsystems limit current to safe levels while still enabling normaloperation of the lead system after MRI or other unsafe events withoutrequiring maintenance of the lead system.

FIG. 4 illustrates an enlarged view of a distal end of a medical deviceelectrical lead in accordance with certain embodiments of the invention.As shown, the electrical lead 70 includes a conductor assembly 72 havingone or more conductors (e.g., such as conductors 74 a and 74 b as shown)covered by an insulating layer 76. As should be appreciated, theconductors 74 a and 74 b are insulated from each other within theconductor assembly 72. Each of the conductors 74 a and 74 b is inelectrical communication with at least one electrode (e.g., such as aring electrode 78 or a tip electrode 80, as respectively shown). Asdescribed above, the electrical lead 70 would function in electricallycoupling a medical device (such as IMDs 10, 30, and 50 of FIGS. 1, 2,and 3, respectively) to the electrodes 78 and 80. Furthermore, theelectrodes 78 and 80 of the lead 70 are placed adjacent or proximate tothe patient's body tissue (as exemplified in FIGS. 1-3) so as to enablethe sensing and/or therapy functioning of the medical device.

While not visibly shown in FIG. 4, a thermally sensitive material(exemplarily illustrated in FIG. 5 and referenced as 82) is electricallyconnected to each of the conductors 74 a and 74 b. The thermallysensitive material 82 is positioned in electrical series between theconductors 74 a and 74 b and their respective electrodes 78 and 80. Assuch, the material 82 forms the only conductive path from the conductors74 a and 74 b to their respective electrodes 78 and 80. As will bedescribed below, the thermally sensitive material 82, in the presence ofunsafe current on the conductors 74 a or 74 b, transitions to a highimpedance state, thereby limiting the current flowing into the patientto be only at safe levels. In certain embodiments, the material 82 ispositioned just before the electrodes 78 and 80 so as to prevent unsafecurrents from being induced within the section of the conductors whichconnect from the material 82 to the electrodes 78 and 80. In certainembodiments, as further illustrated in FIG. 5 and described below, thematerial 82 is integrated in an assembly with each of the electrodes 78and 80; however, the invention should not be limited to such. Forexample, the material 82 can be electrically connected to the electrodes78 and 80 of the lead 70, while being physically separated from theelectrodes 78 and 80.

FIG. 5 shows a cross-sectional view of the ring electrode 78 of themedical device electrical lead 70 of FIG. 4, taken along the lines V-V.As illustrated, FIG. 5 is an axial slice of the ring electrode 78 only;as such, the conductors 74 a and 74 b and the insulating layer 76 arenot shown in the view. In particular, the view illustrates a crosssection of the ring electrode 78 starting from a proximal portion of theelectrode 78 and extending to a distal end of the electrode 78. Asdescribed above with reference to FIG. 4, in certain embodiments, thethermally sensitive material 82 can be integrated with the ringelectrode 78, thereby forming an assembly. In certain embodiments, asshown, the thermally sensitive material 82 is provided as a tube-shapedstructure; however, the invention should not be limited to such.Instead, the material 82 can be provided as any of a wide variety ofdifferent structures (e.g., rings, plates, strips, etc.). In certainembodiments, the material 82 is sized so as to be internally positioned,at least in part, within an outer contact 84 of the ring electrode 78.

As shown, the material 82 is electrically connected to the outer contact84. This connection between the material 82 and the contact 84 can beprovided directly (as exemplarily shown) or indirectly (e.g., via atleast one further contact electrically joining the material 82 and thecontact 84). In certain embodiments, as shown, the distal end portionsof both the material 82 and the contact 84 are positioned so as to be inelectrical contact with each other; however, the point(s) of electricalcontact between the material 82 and the contact 84 can be varied asdesired.

The material 82 is further electrically connected to the correspondingconductor 74 a of the lead 70 (shown in FIG. 4). Similar to thatdescribed above, this connection between the material 82 and theconductor 74 a can be provided directly or indirectly (via at least oneinner contact 86, such as the exemplarily shown weld ring). The innercontact 86 can include any of a variety of different structures. Whilenot shown in FIG. 5, a distal end portion of the lead conductor 74 a forthe ring electrode 78 would be electrically connected to the innercontact 86 (e.g., via a weld joint). In turn, the inner contact 86 wouldbe electrically connected to the material 82, thereby completing theconductive path from the conductor 74 a to the material 82 (via theinner contact 86). In certain embodiments, as shown, a distal endportion of the inner contact 86 and a proximal end portion of thematerial 82 are positioned so as to be in electrical contact with eachother, thereby completing the above-mentioned conductive path; however,the point(s) of electrical contact between the contact 86 and thematerial 82 can be varied to comply with other design requirements.

As should be appreciated, the outer and inner contacts 84, 86 are formedof electrically conductive material, e.g., metal (such as a single metalor a combination of any of a plurality of metals). As described above,in certain embodiments, the distal end portions of both the thermallysensitive material 82 and the outer contact 84 are positioned so as tobe in electrical contact with each other. As further described above, incertain embodiments, a distal end portion of the inner contact 86 and aproximal end portion of the material 82 are positioned so as to be inelectrical contact with each other. As should be appreciated, theseelectrical connections enable a conductive path being established acrossa substantial portion of the material 82 for currents to flow from thelead conductor 78 a to the outer contact 84 of the electrode 78. Assuch, with these electrical connections, the potential for unsafecurrents to arc between the contacts inner contact 86 and the outercontact 84 is restricted, thereby preventing unsafe currents frombypassing the corresponding high impedance pathway through the thermallysensitive material 82.

As illustrated, the ring electrode assembly further includes aninsulating material 88. The insulating material 88 can be formed of anyof a variety of non-conducting materials, such as glass or plastic. Asshown, the insulating material 88 surrounds (e.g., coats) the thermallysensitive material 82 except for the locations in which the outercontact 84 and the inner contact 86 are in electrical contact withthermal sensitive material 82. For example, in certain embodiments, asshown, the insulating material 88 is limited in its coverage of thethermally sensitive material 82 at its distal end (so as to facilitateelectrical connection between the material 82 and the outer contact 84)as well as at its proximal end (so as to facilitate electricalconnection between the material 82 and the inner contact 86).

The inclusion of the insulating material 88 serves at least twofunctions for the electrode assembly. First, the material 88 generallyserves to hermetically seal the thermally insulating material 82 withinthe assembly. Second, the material serves to prevent high currents fromarcing from the lead conductor 74 a (via the inner contact 86) directlyto the outer contact 84. As shown, by surrounding a majority of thethermally sensitive material 82, the insulating material 88 furtherfills in the voids between the inner contact 86 and the outer contact84. In turn, the high currents are further restrained from arcingbetween the inner contact 86 and the outer contact 84, therebyrestricting currents from bypassing the electrical pathway through thethermally sensitive material 82.

While FIG. 5 illustrates a lead design with the thermally sensitivematerial 82 forming an assembly with the ring electrode 78, it should beappreciated that such material 82 can also be likewise incorporated withother electrodes (such as the tip electrode 80) in forming lead designshaving similar current limiting functionality. One skilled in the artwould understand the configuration of these similar designs based onFIG. 5 and the corresponding description above. For example, if thethermally sensitive material 82 were formed with a tip electrode, theouter contact of the electrode, instead of being formed as a ring aroundthe electrical lead, would be affixed at the lead end. Similar to thatdescribed above and shown for the ring electrode 78 of FIGS. 4 and 5,the thermally sensitive material 82 would be sized so as to beinternally positioned, at least in part, within the outer contact of thetip electrode. Further, the material 82 would be surrounded (e.g.,coated) by insulating material (e.g., such as the insulating material 88shown in FIG. 5) except for electrical junction points, facilitatingelectrical connection with a conductor of the electrical lead andfurther electrical connection with the outer contact of the tipelectrode. Another example may involve an electrode having two or moresegmented outer contacts. This would be similar to the above-describedtip electrode configuration, but instead of being affixed at the leadend, the segmented outer contacts would generally be located in one ormore different locations over the length of the lead. In such case, thethermally sensitive material 82 would be accordingly sized andinternally positioned (at least in part) to each of the segmented outercontacts. Further, the material 82 at each outer contact would besurrounded (e.g., coated) by insulating material (e.g., such as theinsulating material 88 shown in FIG. 5) except for electrical junctionpoints, facilitating electrical connection with a conductor of theelectrical lead and further electrical connection with the correspondingsegmented outer contact of the tip electrode.

The above examples describe but two further electrodes that can beconfigured with the thermally sensitive material 82. As should beappreciated and as demonstrated through the use of the above examples,one skilled in the art should be sufficiently equipped from what hasbeen already described and illustrated with respect to the ringelectrode 78 of FIGS. 4 and 5 to apply these teachings in furthervarieties of electrodes commercially available now and in the future.

Additionally, in contrast to the lead design of FIG. 5, whereby thethermally sensitive material 82 is integrated with the ring electrode 78to form an assembly, it is further described herein that the material 82can be separately located outside yet proximate to the ring electrode 78(and tip electrode 80). As such, in certain embodiments, the material 82is electrically connected between the lead conductor 74 a and the ringelectrode 78 and between the lead conductor 74 b and the tip electrode80. Such an embodiment is exemplarily depicted in FIG. 6. As shown, thematerial 82 is positioned adjacent to both the ring electrode 78 and thetip electrode 80. In such cases, the electrical path between thematerial 82 and the corresponding electrodes 78 and 80 is not longenough for dangerous electrical potentials to be induced from the MRIfields.

In certain embodiments, the material 82 is part of an assembly 90. Forexample, in certain embodiments, the material 82 can be hermeticallysealed in insulating material 88 similar to that described above withrespect to the FIG. 5. As should be appreciated, the material 82 can beformed or molded as desired. In certain embodiments, the material 82 isobtained already pre-shaped. In turn, the insulating material 88 can beprovided so as to cover (e.g., coat) the material 82 except for inputand output electrical connections similar to that described above withrespect to FIG. 5. Another exemplary assembly, in certain embodiments,may involve the material 82 being included as a part of a discretethermistor, as further discussed below. It should be appreciated thatthe electrode assembly lead design (of FIG. 5) would provide a morecompact design, though also a more sophisticated design over other leaddesigns having separately positioned elements (depicted in FIG. 6). Oneskilled in the art would understand the functioning of the material 82within such other lead designs based on the description provided abovewith respect to FIG. 5. As such, the functioning of the material 82 insuch lead designs is not further described.

Much of the foregoing description has been concerned with describinglead design configurations that can be provided in utilizing thethermally sensitive material 82. As mentioned above, the thermallysensitive material 82 can be configured to exhibit high impedance in thepresence of unsafe currents. This will limit patient tissue heating.With respect to the material 82, in certain embodiments, a positivetemperature coefficient (PTC) material can be used, as described below.

As is known, a PTC material can be commonly incorporated in thermistors.Thermistors, when placed in an electrical circuit, generally provide achanging resistance with changing temperature of the device. When a PTCmaterial is incorporated in a thermistor, such a PTC thermistor willdemonstrate a sharply increased resistance with increased temperaturefrom a transition temperature. PTC materials are commonly known andcommercially available, such as barium titanate or barium titanate basedmaterials. The resistance/temperature characteristic of the PTCmaterial, whereby the material abruptly transitions to a high impedancestate following such temperature increase from a transition temperature,enables the material to be used in preventing the passage of unsafecurrents there through. While PTC materials are specifically discussedherein, it should be appreciated that any material exhibiting aresistance/temperature characteristic similar to that of PTC materialwould likewise fall within the spirit of the invention.

In use, a PTC thermistor can be configured to exhibit low resistance forcurrents at or below a design point and dramatically higher resistancesat currents beyond the design point. While the PTC material rises intemperature due to such design currents being passed through thematerial, this rise in temperature translates only in a slight increaseto the resistance of the material. As such, the PTC thermistor generallyexhibits adequate electrical conductivity for such design currentlevels. However, if the current flowing into the thermistor exceeds thedesign levels, the temperature of the PTC material will rise above thetransition point. As a result, the electrical resistance of the PTCmaterial will dramatically increase, preventing excessive current fromexcessively heating body tissue. However, when the high potentialcurrent source is removed, the PTC material cools and returns to its lowresistance state, from which design currents can be passed through thePTC thermistor.

The above-described relationship is illustrated in FIG. 7, which shows aresistance versus temperature relationship for a typical PTC thermistormaterial.

Several advantages have been found in using PTC material as thethermally sensitive material 82 described above with respect to the leaddesigns of FIGS. 4 and 5. One advantage in using the PTC material isthat it is not frequency sensitive during times at which it ispreventing flow of unsafe currents to the patient. In particular, whenthe PTC material increases in temperature past its “transitiontemperature” (as further described below), the dielectric constant ofthe material is found to significantly drop (e.g., from about 1000 toabout 1). In turn, capacitive coupling through the PTC material to theelectrodes would not be significant at any of the frequencies generallyassociated with MRI machines. As such, the PTC material can be used withany MRI machine. Another advantage is that for small increases intemperature, the resistance of the PTC material increases by orders ofmagnitude. There is no thermal lag for the PTC material because it isthe temperature of the PTC material that induces the impedancetransition, not the temperature of the adjacent tissue. Consequently,any current flowing into the PTC material is prevented from flowingthrough the electrode and potentially causing damage to body tissueadjacent or proximate to the electrode.

A further advantage is that the PTC material can be precisely adjustedwith respect to the temperature at which its resistivity increases bylarge orders of magnitude, referenced herein as the “transitiontemperature”. Accordingly, this enables the PTC material to be highlyadaptable for use in body-implantable environments. For example, normalbody temperature is 37° C. (or 98.6° F.). As such, the PTC material canbe selected to have a “transition temperature” higher than 37° C.activation temperature (accounting for higher internal body temperaturesof body tissue proximate to the PTC material as well as the temperatureincrease of PTC material for currents higher than nominal or desirablelevels). In certain embodiments, this “transition temperature” can beabout 40° C. (or 104° F.). Another advantage is that PTC thermistormaterials having such a 40° C. “transition temperature” are commerciallyavailable, making such materials less costly and readily available.Furthermore, with respect to using the insulating material 88 togenerally encase the thermally sensitive material 82, as illustrated inFIG. 5, the material 88 functions in further making the material 82 lesssusceptible to the surrounding body tissue cooling the material 82. As aresult, the PTC system is permitted to be more sensitive to electricalcurrent levels and less sensitive to the precision of the transitiontemperature of the PTC material. In addition, the material 82 is lessapt to transmit its heat to the patient's tissue (via the outer contact84). As a result, such a design is highly effective in maintaining thefunctionality of the thermally sensitive material 82 during operation ofthe conductor lead 74 a and corresponding electrode 78, while alsopreventing heating of the patient's tissue proximate to the electrode78.

It will be appreciated the embodiments of the present invention can takemany forms. The true essence and spirit of these embodiments of theinvention are defined in the appended claims, and it is not intended theembodiment of the invention presented herein should limit the scopethereof.

1. An electrical lead, comprising: a conductor assembly having one ormore conductors; an electrode in electrical communication with one ofthe conductors, the electrode having an outer contact adapted forcontacting adjacent body tissue of a patient; and a thermally sensitivematerial electrically connected between the one conductor and theelectrode outer contact, the thermally sensitive material configured toexhibit high impedance in the presence of currents considered unsafe tothe patient, thereby preventing the unsafe currents from flowing throughthe thermally sensitive material and through the electrode outer contactpotentially causing the adjacent body tissue to increase in temperatureto an unsafe level, the unsafe currents causing the thermally sensitivematerial to increase in temperature, thereby causing the material totransition to a high impedance state.
 2. The electrical lead of claim 1,wherein the thermally sensitive material comprises positive temperaturecoefficient material.
 3. The electrical lead of claim 1, wherein thethermally sensitive material is electrically connected in series betweenthe one conductor and the electrode outer contact.
 4. The electricallead of claim 1, wherein the thermally sensitive material is integratedinto an assembly with the electrode.
 5. The electrical lead of claim 4,wherein the thermally sensitive material is sized so as to be internallypositioned at least partially within the electrode.
 6. The electricallead of claim 4, wherein the one conductor is electrically coupled to aproximal end portion of the thermally sensitive material and theelectrode outer contact is electrically coupled to a distal end portionof the thermally sensitive material, wherein a conductive path betweenthe one conductor and the electrode outer contact is established acrossa substantial portion of the thermally sensitive material.
 7. Theelectrical lead of claim 4, wherein the assembly further comprises aninsulating material enclosing a majority of an outer surface of thethermally sensitive material, thereby enhancing a conductive pathbetween the one conductor and the electrode outer contact across thethermally sensitive material.
 8. The electrical lead of claim 1, whereinthe thermally sensitive material is physically separate from theelectrode.
 9. The electrical lead of claim 8, wherein the thermallysensitive material is included as a part of a discrete positivetemperature coefficient thermistor.
 10. An electrode, comprising: anouter contact adapted for contacting adjacent body tissue of a patient;an inner contact adapted for electrical coupling to a lead conductor;and a thermally sensitive material electrically connected between theinner contact and the outer contact, the thermally sensitive materialconfigured to exhibit high impedance in the presence of currentsconsidered unsafe to the patient, thereby preventing the unsafe currentsfrom flowing through the thermally sensitive material and through theouter contact potentially causing the adjacent body tissue to increasein temperature to an unsafe level, the unsafe currents causing thethermally sensitive material to increase in temperature, thereby causingthe material to transition to a high impedance state.
 11. The electrodeof claim 10, wherein the thermally sensitive material comprises positivetemperature coefficient material.
 12. The electrode of claim 10, whereinthe thermally sensitive material is electrically connected in seriesbetween the inner contact and the outer contact.
 13. The electrode ofclaim 12, wherein the thermally sensitive material is sized so as to beinternally positioned at least partially within the outer contact. 14.The electrode of claim 12, wherein the inner contact is electricallycoupled to a proximal end portion of the thermally sensitive materialand the outer contact is electrically coupled to a distal end portion ofthe thermally sensitive material, wherein a conductive path between theinner contact and the outer contact is established across a substantialportion of the thermally sensitive material.
 15. The electrode of claim12, wherein the assembly further comprises an insulating materialenclosing a majority of an outer surface of the thermally sensitivematerial, thereby enhancing a conductive path between the inner contactand the outer contact across the thermally sensitive material.
 16. Amethod of providing an electrical lead configured to prevent currentsconsidered unsafe to a patient, comprising: providing a conductorassembly having one or more conductors; providing an electrode inelectrical communication with one of the conductors, the electrodehaving an outer contact adapted for contacting adjacent body tissue of apatient; and electrically connecting a thermally sensitive materialbetween the one conductor and the electrode outer contact, the thermallysensitive material configured to exhibit high impedance in the presenceof currents considered unsafe to the patient, thereby preventing theunsafe currents from flowing through the thermally sensitive materialand through the electrode outer contact potentially causing the adjacentbody tissue to increase in temperature to an unsafe level, the unsafecurrents causing the thermally sensitive material to increase intemperature, thereby causing the material to transition to a highimpedance state.
 17. The method of claim 16, wherein the thermallysensitive material upon reaching a certain temperature exhibitssignificant increased resistance upon further increases in temperature.18. The method of claim 16, wherein the thermally sensitive material iselectrically connected in series between the one conductor and theelectrode outer contact.
 19. The method of claim 16, wherein thethermally sensitive material is integrated into an assembly with theelectrode.
 20. The method of claim 19, wherein the thermally sensitivematerial is sized so as to be internally positioned at least partiallywithin the electrode.
 21. The method of claim 19, wherein the oneconductor is electrically coupled to a proximal end portion of thethermally sensitive material and the electrode outer contact iselectrically coupled to a distal end portion of the thermally sensitivematerial, wherein a conductive path between the one conductor and theelectrode outer contact is established across a substantial portion ofthe thermally sensitive material.
 22. The method of claim 19, whereinthe assembly further comprises an insulating material enclosing amajority of an outer surface of the thermally sensitive material,thereby enhancing a conductive path between the one conductor and theelectrode outer contact across the thermally sensitive material.
 23. Themethod of claim 16, wherein the thermally sensitive material isphysically separate from the electrode.
 24. The method of claim 23,wherein the thermally sensitive material is included as a part of adiscrete positive temperature coefficient thermistor.
 25. An electricallead, comprising: a conductor assembly having one or more conductors; anelectrode in electrical communication with one of the conductors, theelectrode having an outer contact adapted for contacting adjacent bodytissue of a patient; and a positive temperature coefficient materialelectrically connected between the one conductor and the electrode outercontact, the positive temperature coefficient material integrated intoan assembly with the electrode, the positive temperature coefficientmaterial configured to exhibit high impedance in the presence ofcurrents considered unsafe to the patient, thereby preventing the unsafecurrents from flowing through the positive temperature coefficientmaterial and through the electrode outer contact potentially causing theadjacent body tissue to increase in temperature to an unsafe level, theunsafe currents causing the positive temperature coefficient material toincrease in temperature, thereby causing the material to transition to ahigh impedance state.
 26. The electrical lead of claim 25, wherein thepositive temperature coefficient material is electrically connected inseries between the one conductor and the electrode outer contact. 27.The electrical lead of claim 25, wherein the positive temperaturecoefficient material is sized so as to be internally positioned at leastpartially within the electrode.
 28. The electrical lead of claim 25,wherein the one conductor is electrically coupled to a proximal endportion of the positive temperature coefficient material and theelectrode outer contact is electrically coupled to a distal end portionof the positive temperature coefficient material, wherein a conductivepath between the one conductor and the electrode outer contact isestablished across a substantial portion of the positive temperaturecoefficient material.
 29. The electrical lead of claim 25, wherein theassembly further comprises an insulating material enclosing a majorityof the outer surface of the positive temperature coefficient material,thereby enhancing a conductive path between the one conductor and theelectrode outer contact across the positive temperature coefficientmaterial.